This invention relates to an assembly for capping a fuel inlet portion of a fuel storage tank provided for a motor vehicle or the like and more particularly, to a type of capping assembly that is provided with a valve arrangement, which is capable of controlling the pressure inside the fuel storage tank regardless of variations in that pressure.
Presently, there have been proposed a number of capping assemblies for use in fuel storage tanks, each being specifically arranged to cope with a specific problem wherein when a flow rate of the fuel from the tank is decreased, either when the amount of the fuel in the tank is considerably decreased or, when a comparatively large difference in temperature between the inside of the tank and the outside of the tank is present. According to such conventional assembles as described above, an internal passage is formed inside the assembly and a valve arrangement is conventionally incorporated into the internal passage in a manner such that when the pressure inside the tank is lowered to a predetermined pressure below the atmospheric pressure, the valve arrangement is correspondingly opened so as to permit air having the atmospheric pressure to flow into the inside of the tank. More specifically, referring now to FIG. 1, there is shown a typical capping assembly of the above-described conventional type, which is mounted on a fuel inlet portion of a fuel storage tank 11. The assembly comprises a cover 1, a casing 2, and a valve arrangement 3, and the valve arrangement 3 including a valve 6, a coil spring 10 and a disc member 8 is incorporated in the casing 2. The casing 2 is provided with an aperture 4 at the central bottom portion thereof, while the circumferential portion of the aperture 4 is backed by a sheet member 5 of rubber material. The valve 6 is arranged to be generally accommodated inside the casing 2, while a rod portion 7 is extended through the aperture 4. A substantially W-shaped disc member 8 is securedly mounted on an end of the rod portion 7 in a manner such that an outermost circumferential portion of the disc member 8 can selectively contact the sheet member 5 in association with a vertical movement of the valve. A flange 9 is mounted on the top end of the rod portion 7. Furthermore, the coil spring 10, of the compression type, is positioned between the flange portion 9 and the portion defining the outer periphery of the aperture 4, and the rod portion 7 is enclosed by the coil spring 10. Accordingly, as long as an external force is not applied onto the valve arrangement 3, the outermost circumferential portion of the disc member 8 is in contact with the sheet member 5 by the spring force of the coil spring 10, and thus, the inside of the fuel storage tank is shut off from the atmosphere. On the other hand, when the pressure inside the fuel storage tank 11 is lowered to a predetermined pressure below the atmospheric pressure, the disc member 8 is drawn toward the inside of the tank. Such being the case and since air of atmospheric pressure is introduced, through a clearance provided between the cover 1 and the casing 2 and between the sheet member 5 and the outermost circumferential portion of the disc member 8 to the inside of the tank, the pressure inside the tank is raised, to atmospheric. Thus, the pressure inside the tank can be maintained at a predetermined pressure level according to the conventional assembly.
However, the conventional assembly of the above-described type has the following defects. One of the defects can be referred to an arrangement in which the outermost circumferential portion of the disc member 8 is normally urged toward the sheet member 5. Namely, the assembly can not have the advantage of rendering a high sealing capability, since the sealing area between the outermost circumferential portion of the disc member 8 and the sheet member 5 is comparatively large relative to the total force exerted by the coil spring. Namely, according to the capping assembly of the pressure controlling type, the valve arrangement is required to respond to a change of pressure inside the tank in quite a sensitive manner. Therefore, with such a valve arrangement as shown in FIG. 1, either the coil spring having a relatively small spring force must be used in the arrangement, or the circumferential portion of the disc member has to be made comparatively large. However, if the valve arrangement is arranged to satisfy the latter condition, the sealing pressure per unit sealing area is correspondingly decreased, and thus, the resultant assembly can not in turn have the advantage of having a high sealing capability. Furthermore, the outermost circumferential portion of the disc member 8 requires a precise finish so that an excellent sealing condition between the disc member 8 and the sheet portion 5 can be expected. In addition, another defect is related to the fact that the outermost circumferential portion of the disc member 8 tends to adhere to the surface of the sheet member 5, since the sheet member 5 is conventionally made of a rather sticky rubber. Such being the case, even if the pressure inside the tank 11 is negative relative to the predetermined pressure, the outermost circumferential portion will still remain in a state adhering to the surface of the sheet member 5 as described above, and thus, the valve arrangement 3 can not function properly.